The principal objective of the proposed research is to elucidate themechanism of vascular smooth muscle relaxation by nitorgen oxide-containing vasodilator drugs and to study the relationships between these drugs on the one hand and nitric oxide (NO), S-nitrosothiols and cyclic GMP on the other. The mechanisms by which these clinically established vasodilators (glyceryl trinitrate, nitroprusside, amyl nitrate) relax vascular smooth muscle are completely unknown. Evidence is amassing that tissue -SH groups and cyclic GMP are required for the expression of their relaxant responses. We have demonstrated the dependence on thiols (-SH) of coronary arterial guanylate cyclase activation by these vasodilator drugs. Moreover, we have recently found that nitrogen oxide-containing drugs can releas or form NO, which reacts with thiols toform S-nitrosothiols. These S-nitrosothiols are potent vascular smooth muscle relaxants, markedly activate guanylate cyclase and elevate smooth muscle levels of cyclic GMP. The specific aims of the proposal are: 1) to compare the effects of several classes of vasodilator drug on bovine coronary artery, intrapulmonary artery and intrapulmonary vein; 2) to compare cardiovascular hemodynamic effects of the drugs and S-nitrosothiols in cats and dogs; 3) to study S-nitrosothiol formation from vascular tissue and thiols in the presence of vasodilator drugs; 4) to explore the role of cyclic GMP in modulating the relaxant effects of drugs and S-nitrosothiols; 5) to determine the mechanism of tolerance to organic nitrates and its relationship to S-nitrosothiol and cyclic GMP formation; 6) to ascertain the mechanism of activation of purified guanylate cyclase by nitrogen oxide vasodilators, NO and S-nitrosothiols. The proposed research represets a novel approach to a long-standing problem. New information from these studies should enable us to better understand both the desirable and undesirable effects of these vasodilators, and, therefore, should provide us with a rational means for modifying the drugs chemically and/or altering modes of drug therapy clinically.